Due to my symptoms, my psychiatrist said that my serotonin has decreased. Is there any solution to this other than medication?

Possibly. Discussed in past questions: depression benefits from exercise, healthy diet, stress reduction and psychotherapy to change one's perception of negative situations. Unfortunately when depression is severe pts motivation to do these things or think clearly are impaired and meds are necessary.
It depends. If you are under stress, your body needs extra seretonin and your body is already depleated and can not keep up with the demand that you need. You need more. As you need more and can't keep up your body needs even more. It is a vicious cycle. Medication helps so that your body can not only use what you are making but get the extra that you need to help you through the hard times.